This invention relates to a cathode-ray tube (CRT) and, more particularly to a color CRT including a tension focus mask.
A color cathode-ray tube (CRT) typically includes an electron gun, an aperture mask-frame assembly, and a screen. The aperture mask-frame assembly is interposed between the electron gun and the screen. The screen is located on an inner surface of a faceplate of the CRT tube. The screen has an array of three different color-emitting phosphors (e.g., green, blue and red) formed thereon. The aperture mask functions to collimate the electron beams generated in the electron gun toward appropriate color-emitting phosphors on the screen of the CRT.
The aperture mask may be a focus mask. Focus masks typically comprise two sets of electrodes that are arranged orthogonal to each other, to form an array of openings. Different voltages are applied to the two sets of electrodes so as to create quadrupole focusing lenses in each opening of the mask, which are used to direct and focus the electron beams toward the appropriate color-emitting phosphors on the screen of the CRT tube.
One type of focus mask is a tension focus mask, wherein at least one of the sets of electrodes is under tension. Typically, for tension focus masks, the vertical electrodes are held in tension by the mask frame. The other set of electrodes is horizontal and overlays the vertical electrodes, which are typically strands. An etching process used on a flat sheet of metal commonly forms the strands. Such an etching process forms sharp corner edges along the length of the strands.
The two sets of electrodes overlap at a series of points known as junctions. At these junctions the individual elements of one set of electrodes are separated from the individual elements of the other set by an insulating material. When the different voltages are applied between the two sets of strands of the mask, to create the quadrupole focusing lenses in the openings thereof, surface flashover may occur at one or more of the junctions. Surface flashover is a breakdown process that may take place on or near the surface of the insulating material separating the two sets of strands and may lead to arcing between the strands at one or more places on the focus mask. Since the overlying wires are electrically connected to one another, all of the energy stored in the capacitance of the entire focus mask is available to arc. This stored energy may be sufficient to cause local melting of the strands and/or the insulating material and may result in an electrical short leading to the subsequent failure of the focus mask. Surface flashover has a greater risk of occurring in locations in which one of the electrodes has a sharp edge, since the local electric field can be higher at these locations.
Additionally, during operation of the CRT tube, electron scattering may occur along sharp edges of the mask strands. Electron scattering along strand edges of the focus mask is undesirable because some of these electrons may strike the wrong color element, degrading the color purity of the CRT tube.
Thus, a need exists for suitable tension focus masks that overcome the above-mentioned drawbacks.
The present invention relates to a color cathode-ray tube (CRT) having an evacuated envelope with an electron gun therein for generating at least one electron beam. The envelope further includes a faceplate panel having a luminescent screen with phosphor lines on an interior surface thereof. A tension focus mask, having a plurality of spaced-apart first conductive electrodes, is located generally parallel to an effective picture area of the screen. The plurality of spaced-apart first conductive electrodes, otherwise known as strands, have a screen-facing side and electron-gun facing side. Each side of the strands have sharp corner edges extending along the length of the strands. A plurality of second conductive electrodes are oriented substantially perpendicular to the plurality of strands and separated by an insulating material deposited on the screen-facing side and corners of the strands to shield the sharp edges of the strands from the second conductive electrodes. In doing so, the present invention reduces the risk of surface flashover that would occur when sharp corners are formed using prior art etching processes.